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Background: Radiotherapy is one of the cancer treatment methods. Prescribed dose for each fraction is considered based on radiosensitivity of tumoral and normal tissues. Viral agents are the effiectiv factors on tissue sensitivity. This research aimed to determine the effect of ionizing radiation of Cobal 60 on radiosensitivity of Hela cells infected with Measles virus. Methods and Materials: In this study, the radiosensitivity of Hela cells is investigated experimentally and qualitively. The cells have been cultivated in two groups (experimental and blank) and plating efficiency has been obtained. Then 100λ measles virus with serial dilution method was used to induce infection in different ratio for experimental group. After cell growth and passage, the two groups were irradiated with 2Gy gamma radiation of cobalt 60. Results: Results respectively indicated cell death increases up to 5-7%, 15-20% and 50-65%, after 2Gy irradiation by Co 60 for contaminating to Measles in low, moderate and high concentrations. Conclusion: Radiosensitivity of tumoral cells increases when they are infected by viral agent. The result in radiotherapy of cancers showed, in prescribing dose fraction non cancer disease should be considered.